Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the echo return. Conventional B-mode images are formed from a combination of fundamental and harmonic signal components, the former being direct echoes of the transmitted pulse and the latter being generated in a nonlinear medium such as tissue from finite-amplitude ultrasound propagation. In certain instances, e.g., obese patients, ultrasound images can be improved by suppressing the fundamental and emphasizing the harmonic signal components.
Tissue harmonic imaging was proposed in an article by Averkiou et al. entitled, "A new imaging technique based on the nonlinear properties of tissues," Proc. 1997 IEEE Ultrasonic Symp. Propagation of sound beams in biological tissues is known to be nonlinear, giving rise to the generation of harmonics. In one type of tissue harmonic imaging, energy is transmitted at a fundamental frequency .function..sub.0 and an image is formed with energy at the second harmonic 2.function..sub.0. Some of the characteristics of the nonlinearly generated second harmonic beams are a narrower beam, lower sidelobes than the fundamental and beam formation in a cumulative process, i.e., the second harmonic continually draws energy from the fundamental during propagation. These characteristics contribute to lateral resolution improvements, reduction of multiple reflections due to tough windows, and clutter reduction due to inhomogeneities in the tissue and skin layers.
Contrast agents have been developed for medical ultrasound to aid in diagnosis of traditionally difficult-to-image vascular anatomy. For example, the use of contrast agents is discussed by de Jong et al. in "Principles and Recent Developments in Ultrasound Contrast Agents," Ultrasonics, Vol. 29, pp. 324-380 (1991). The agents, which are typically microbubbles whose diameter is in the range of 1-10 micrometers, are injected into the blood stream. Since the backscatter signal of the microbubbles is much larger than that of blood cells, the microbubbles are used as markers to allow imaging of blood flow. One method to further isolate echoes from these agents is to use the (sub)harmonic components of the contrast echo, which is much larger than the harmonic components of the surrounding tissue without contrast agent. [See, e.g., Newhouse et al., "Second Harmonic Doppler Ultrasound Blood Perfusion Measurement," Proc. 1992 IEEE Ultrason. Symp., pp. 1175-1177; and Burns, et al., "Harmonic Power Mode Doppler Using Microbubble Contrast Agents: An Improved Method for Small Vessel Flow Imaging," Proc. 1994 IEEE Ultrason. Symp., pp. 1547-1550.]
At least two methods for harmonic imaging in an ultrasound scanner are known. In the first method, the transducer elements of a phased array are activated by waveforms having a fundamental frequency and time-delayed to produce an ultrasound beam which is focused at a transmit focal zone, transmission of a single focused beam being referred to as a "firing". The echoes returned from the body being interrogated are transduced by the array elements into electrical signals and time-delayed to form a receive vector of acoustic data having both fundamental and harmonic signal components. The receive filter removes the fundamental signal component and isolates the harmonics signal component. The latter component is then detected, scan-converted and displayed.
In the second method, each transducer element is activated by a first phase-encoded waveform having one polarity during a first transmit firing and by a second phase-encoded waveform having the opposite polarity during a second transmit firing. Both waveforms have a fundamental frequency. The activations of the transducer elements during each firing are time-delayed to produce an ultrasound beam which is focused at the same transmit focal zone. Each firing results in a respective receive vector of acoustic data, each vector having both fundamental and harmonic signal components. When the receive vectors are vector summed, however, the fundamental signal components substantially cancel, thereby isolating a harmonic signal component. The latter component is then detected, scan-converted and displayed.
The problems with the first method include the following: (a) the received signal is narrowband and hence resolution is poor; (b) it is very difficult to filter the large fundamental signal component completely, so there is some residual fundamental signal that degrades contrast improvement; and (c) if the transmit signal contains harmonic frequencies, it is not possible to filter out that component.
The second method is not afflicted by the foregoing disadvantages of the first method. However, a major drawback of the second method is that it requires two firings to acquire harmonic data corresponding to a particular transmit focal zone and hence always decreases the frame rate by half. For lower-frequency transducers, where imaging is done to depths of 20-24 cm, the second method is often not realizable.